In a small bowl, cream the butter, brown sugar, peanut butter and vanilla until light and fluffy. Gradually add flour until well combined (mixture will be crumbly).
Press onto the bottom and 1/2 in. up the sides of an 8x4-in. loaf pan coated with cooking spray. Bake at 350° for 12-15 minutes or until set and lightly browned. Cool.
For filling, in a small saucepan, cook and stir the milk and butter over low heat until mixture comes to a boil and thickens. Remove from the heat; stir in peanuts and vanilla. Spread evenly over crust.
Bake at 350° for 15-17 minutes or until set. Immediately sprinkle with chocolate chips; let stand for a few minutes to soften, then spread over filling. Sprinkle with peanuts. Cool completely on a wire rack. Cut into bars.